Observation of two charge ordering transitions in the valence-fluctuating EuPtP by resonant x-ray diffraction

Abstract

Two first-order phase transitions of the valence-fluctuating compound EuPtP at T 1 = 246 K and T 2 = 200 K were examined by means of resonant x-ray diffraction at the Eu L 3 absorption edge. Although previous Mössbauer spectroscopy and preliminary x-ray diffraction experiments have suggested chargeordered states, characteristic energy spectra of the 111 and 11 2 3 reflections observed in the present experiment unambiguously indicated that twofold and threefold superstructures of Eu valence exist below T 2 and between T 1 and T 2 , respectively. We also propose a mechanism to explain the charge ordering in EuPtP based on a charge-lattice coupling that alters a Coulomb repulsion.

Valence ordering in the intermediate-valence magnet YbPd

“…18,19 The ANNNI model has been applied to 4f electron magnets including CeSb, 20 CeBi, 21 and UPd 2 Si 2 , 22 to ferroelectrics such as NaNO 2 23 and [N(CH 3 ) 4 ] 2 MCl 4 , 24 and to charge/valence ordering systems such as NaV 2 O 5 25 and EuPtP. 5 Among these materials, EuPtP has significant similarities to YbPd, because both are metallic valence-fluctuating compounds.…”

“…Charge ordering is one of the most widely studied phenomena in transition-metal oxides, 1 organic conductors, 2 and rareearth compounds. [3][4][5] Normally, charge ordering is seen in materials with low conductivity, and the phenomenon involves a sudden increase in the electrical resistivity. However, YbPd is a good metal in all temperature ranges 6 with resistivity of ∼10 2 μ cm, and is proposed to have a charge order accompanied by a decrease in resistivity.…”

“…Using the L I I I absorption edge energy, it is possible to study the valence arrangements of 4f electron systems. 4,5 We have succeeded in clearly observing the superstructure induced by valence ordering in YbPd by means of these bulk-sensitive techniques.…”

NMR studies of ordered structures and valence states in the successive valence-transition system EuPtP

“…The drops of the susceptibility and of the unit cell volume both just below T A and T B are consistent with an increase in Eu 3+ component [5,6]: Eu 3+ state is nonmagnetic, and the lattice generally shrinks with smaller Eu 3+ ions than Eu 2+ ions. Indeed, the reports of the Eu L III edge x-ray absorption spectroscopy (XAS) indicate that the Eu valence changes at T A and T B [7,9], that is, the mean values are estimated as 2.10 for T > T A (denoted as α phase), 2.33 for T B < T < T A (β phase), and 2.41 for T < T B (γ phase) [9]. Moreover, EuPtP exhibits antiferromagnetic ordering at 8.6 K [5,7,8].…”

“…There are also several works which attempted to identify ordered structures in the β and γ phases [7,9]. However, this issue has not been fully confirmed yet.…”